Glazing made from compositions containing transparent thermoplastic polymers such as, for example, polycarbonate offer many advantages over conventional glazing made of glass for use in the automotive sector and for buildings. Such advantages include, for example, increased break resistance and/or an increased weight saving, which in the case of automotive glazing permit greater safety for the occupants in the event of road traffic accidents and a lower fuel consumption. Finally, transparent materials containing transparent thermoplastic polymers permit substantially greater freedom in terms of design because they are easier to mould.
It is a disadvantage, however, that the high heat transmissibility (i.e. transmissibility for IR radiation) of transparent thermoplastic polymers leads to undesirable heating of the inside of motor vehicles and buildings under the action of the sun. The raised temperatures on the inside reduce the comfort for the occupants or residents and can involve increased demands in terms of air conditioning, which in turn increase the energy consumption and thus eliminate the positive effects again. In order nevertheless to meet the demand for low energy consumption coupled with a high degree of comfort for the occupants, glazing provided with appropriate heat protection is required. This is true for the automotive sector in particular.
As has long been known, the largest part of solar energy, apart from the visible range of light between 400 nm and 750 nm, is accounted for by the near-infrared (NIR) range between 750 nm and 2500 nm. Penetrating solar radiation is absorbed inside a car, for example, and emitted as long-wave heat radiation having a wavelength of from 5 μm to 15 μm. Because conventional glazing materials—in particular thermoplastic polymers that are transparent in the visible range—are not transparent in that range, the heat radiation is unable to radiate to the outside. A greenhouse effect is obtained and the interior heats up. In order to keep this effect to a minimum, the transmission of the glazing in the NIR should be minimised as far as possible. Conventional transparent thermoplastic polymers such as, for example, polycarbonate are, however, transparent both in the visible range and in the NIR.
Additives, for example, which exhibit as low a transparency as possible in the NIR without adversely affecting the transparency in the visible range of the spectrum are therefore required.
Of the transparent thermoplastic plastics, polymers based on polymethyl methacrylate (PMMA) and polycarbonate are particularly suitable for use as a glazing material. Because of its high strength, polycarbonate in particular has a very good property profile for such uses.
In order to impart infrared-absorbing properties to these plastics, corresponding infrared absorbers are used as additives. IR absorber systems which have a broad absorption spectrum in the NIR range (near-infrared, 750 nm-2500 nm) while at the same time having low absorption in the visible range (low inherent colour) are of particular interest for that purpose. The corresponding polymer compositions should additionally have high heat stability as well as excellent light stability.
A large number of IR absorbers based on organic or inorganic materials which can be used in transparent thermoplastics are known. A selection of such materials is described, for example, in J. Fabian, H. Nakazumi, H. Matsuoka, Chem. Rev. 92, 1197 (1992), in U.S. Pat. No. 5,712,332 or JP-A 06240146.
IR-absorbing additives based on organic materials frequently have the disadvantage, however, that they exhibit poor stability towards thermal stress or radiation. Accordingly, many of these additives do not have sufficient heat stability to be incorporated into transparent thermoplastics because temperatures of up to 350° C. are required for their processing. Moreover, during use, glazing is often exposed for prolonged periods to temperatures of more than 50° C., caused by solar radiation, which can lead to decomposition or degradation of the organic absorbents.
Furthermore, organic IR absorbers frequently do not have a sufficiently broad absorption band in the NIR range, so that their use as IR absorbers in glazing materials is inefficient, a pronounced inherent colour of such systems often also occurring, which is generally undesirable.
IR-absorbing additives based on inorganic materials are frequently markedly more stable as compared with organic additives. The use of such systems is often also more economical because in most cases they have a markedly more favourable price/performance ratio. Accordingly, materials based on finely divided borides, such as, for example, lanthanum hexaboride, have proved to be efficient IR absorbers because, in particular, they have a broad absorption band. Such borides based on La, Ce, Pr, Nd, Tb, Dy, Ho, Y, Sm, Eu, Er, Tm, Yb, Lu, Sr, Ti, Zr, Hf, V, Ta, Cr, Mo, W and Ca are described inter alga, for example, in DE 10392543 or EP 1 559 743.
However, a disadvantage of these additives is their significant inherent colour. After incorporation, these boride-containing additives impart a characteristic green colour to the transparent plastic, which is frequently undesirable because it greatly limits the scope for a neutral colouring.
In order to compensate for the inherent colour, relatively large amounts of further colouring agents are often used, but this impairs the optical properties of the composition and leads to markedly reduced transmission in the visible range. This is undesirable in the case of motor vehicle glazing in particular or, in special cases in which the driver's view must not be impaired, is inadmissible.
It has been shown that IR-absorbing additives from the group of the tungstates have a lower intrinsic absorption in the visible spectral range as compared with the boride-based inorganic IR absorbers known from the prior art and yield thermoplastic materials having a low inherent colour. Moreover, they have a desirable broad absorption characteristic in the NIR range. Such tungstates are tungsten oxides based on WyOz (W=tungsten, O=oxygen, z/y=2.20-2.99) or based on MxWyOz (M=H, He, alkali metal, alkaline earth metal, rare earths, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, Be, Hf, Os, Bi; x/y=0.001-1, z/y=2.2-3.0).
The preparation and use of these substances in thermoplastic materials is known in principle and described, for example, in H. Takeda, K. Adachi, J. Am. Ceram. Soc. 90, 4059-4061, (2007), WO 2005037932, JP 2006219662, JP 2008024902, JP 2008150548, WO 2009/059901 and JP 2008214596.
The low inherent colouring of these systems is advantageous because it permits a large degree of freedom in the colouring of the end product. The final colour is achieved by adding further pigments or colourants. Furthermore, the IR-absorbing polycarbonate compositions exhibit an economically acceptable or even valuable price/performance ratio.
The IR-absorbing additives from the group of the tungstates are suitable in principle for transparent thermoplastics such as polymethyl methacrylate and polycarbonate on account of the advantages described above. However, it has been shown that these additives lead to unexpected colour impressions in transparent thermoplastic compositions, irrespective of their inherent colour.
The colour impression of a non-transparent object is attributable to the reflected light. An object which, for example, absorbs the long-wave constituents of light appears blue because the remaining shorter-wave components of the spectrum are remitted. However, this application relates to transparent objects, such as, for example, window panels. Transparent objects in this application are understood as being bodies that exhibit a transmission (unweathered, not aged) of at least 10% and a haze of less than 3.0%, preferably less than 2.5%, more preferably less than 2.0% and particularly preferably less than 1.5%. In the case of transparent bodies, in contrast to non-transparent objects, it is normally not the remitted colour but the transmitted colour that is in the foreground. The object thus acts as a colour filter. In order not to impair the transparency of the panel there are preferably used colouring agents which dissolve in the polymer matrix or have such a small particle size that they cause no haze, no haze within the scope of the present invention meaning a haze of less than 3% at a given layer thickness, measured in accordance with ASTM D1003.
The tungstate-based IR absorber particles that are used do not in fact lead to haze of the corresponding glazing element (haze <3%).
However, it has been found that, above a certain concentration, these particles, whose size is preferably within the nanometre range, can cause scattering effects in the matrix in which they are embedded, regardless of the nature and other properties of the particles. While this scattering has only an unnoticeable effect on the transmission and accordingly the transparency of the article, the colour impression of the article is in some cases changed considerably by the scattered light, in particular in dependence on the viewing angle.
Consequently, the IR-absorbing additives from the group of the tungstates accordingly lead to undesirable colour reflexes in the finished part, that is to say, for example, in a transparent panel, under certain light conditions and viewing angles. Thus, corresponding panels exhibit a bluish to violet tinge according to the concentration of the inorganic IR absorber used. This colour impression does not arise from the inherent colour of the chosen added pigments and absorbers but is attributable to scattering effects of the nanoparticles, which are to be observed in particular at viewing angles of from 1 to 60°, which lie outside the angle of reflection. Such scattering can adversely affect the overall colour impression of the corresponding article, for example a vehicle or a building.
The scattering effect is, as described, frequently perceived as a bluish-violet colour impression. A neutral colour impression is frequently desirable, that is to say the natural colour impression is not disturbed by scattering effects. This means that the colour produced by the scattering effect must on the one hand be relatively close to the achromatic point and on the other hand close to the inherent colour of the component.
It must be emphasised that this colour effect does not result from the normal absorbed or transmitted colour. This phenomenon is only caused by scattered light. Colourants or colouring pigments do not normally contribute to this colour effect. Only certain additives, such as, for example, the nano-scale tungsten-based IR absorbers, cause this effect. Furthermore, it must be pointed out that the scattering effect is pronounced only under certain light conditions and defined viewing angles. This is the case, for example, when the article—preferably a panel—is viewed under good light conditions, that is to say under solar radiation and at observation angles of from 1 to 60°.
The bluish scattering is caused by the IR additive, which consists of fine particles. These particles, which on average have a size, which can be determined, for example, by means of TEM (transmission electron microscopy), of preferably less than 200 nm, particularly preferably less than 100 nm, cause a scattering effect and can accordingly also lead to undesirable colour impressions. In order to minimise that effect, the diameter of the particles could be reduced or the amount of particles in the matrix could be limited. However, this is complex because either the particles must be very finely ground and the risk of reagglomeration exists or, if the particle concentration is too low, the desired effect of IR absorption can no longer be achieved.
It is known that finely divided particles can cause so-called Rayleigh scattering. This Rayleigh scattering is described, for example, in C. F. Bohren, D. Huffmann, Absorption and scattering of light by small particles, John Wiley, New York 1983. The scattering behaviour of tungstate-based nanoparticles has not hitherto been described. The concentration ranges in which the described colour scattering effects occur in a thermoplastic matrix were likewise hitherto not known. Measures for attenuating the described effect were not obvious from the current prior art.
Further, thermoplastic moulding compositions are known which contain both IR absorbers and colouring pigments, inter alia carbon blacks, in order to influence both the heat-absorbing properties and the colouration. However, measures for reducing the scattered radiation caused by tungstate-based IR-absorbing particles are as rarely described in the literature as that undesirable effect.
Compositions based on polycarbonate containing tungstate-based inorganic IR absorbers are described in various publications.
Moulding compositions containing boride-based IR absorbers and specific carbon blacks are known from WO 2007/008476 A1, a synergistic effect in respect of the IR-absorbing properties being said to be achieved by the combination of these components.
US 2006/0251996 describes inorganic IR absorbers, including inter alia also tungstates, which can be used in conjunction with further pigments, for example also carbon black. However, neither the problem of colour scattering nor compositions having a specific ratio of IR absorber to carbon black are described in US 2006/0251996.
None of the documents described as prior art describes scattering or reflection effects of inorganic IR absorbers in thermoplastics and the problems resulting therefrom.